Rare-earth magnets that use rare-earth elements, such as lanthanoid, are also called permanent magnets. Such magnets are used not only for hard disks or motors of MRI but also for driving motors of hybrid vehicles, electric vehicles, and the like.
As examples of magnetic performance indices of such rare-earth magnet, residual magnetization (i.e., residual magnetic flux density) and coercivity can be given. However, with a reduction in the motor size and an increase in the amount of heat generation that has been achieved with an increase in the current density, there has been an increasing demand for higher heat resistance of the rare-earth magnet being used. Thus, how to retain the coercivity of a magnet under high-temperature use environments is an important research object to be achieved in the technical field. For example, for a Nd—Fe—B-based magnet, which is one of the rare-earth magnets that are frequently used for vehicle driving motors, attempts have been made to increase the coercivity by, for example, reducing the crystal grain size, using an alloy with a high Nd content, or adding a heavy rare-earth element with high coercivity performance, such as Dy or Tb.
Examples of rare-earth magnets include typical sintered magnets whose crystal grains (i.e., a main phase) that form the structure have a scale of about 3 to 5 and nanocrystalline magnets whose crystal grains have been reduced in size down to a nano-scale of about 50 to 300 nm. Among them, nanocrystalline magnets for which the amount of addition of an expensive heavy rare-earth element can be reduced (i.e., reduced to zero) while the crystal grain size can also be reduced as described above are currently attracting attention.
The resource cost of Dy, which is frequently used among heavy rare-earth elements, has been rapidly increasing since the Japanese fiscal year 2011 as the prospecting areas of Dy are mostly distributed in China and the amount of production as well as the amount of exports of rare metals, such as Dy, by China is now regulated. Therefore, development of a magnet with a less Dy content, which has a reduced Dy content but has ensured coercive performance, and a Dy-free magnet, which contains no Dy but has ensured coercive performance, is one of the important development tasks to be achieved, and this has been one of the factors that are increasing the degree of attention of nanocrystalline magnets.
A method for producing a nanocrystalline magnet is briefly described below. For example, a sintered body is produced by sintering nano-sized fine powder, which has been obtained through liquid quenching of a melt of a Nd—Fe—B-based metal, while at the same time performing pressure molding, and then performing hot plastic processing to the sintered body to impart magnetic anisotropy thereto, whereby a molded body is produced.
A heavy rare-earth element with high coercivity performance is imparted to such a molded body by various methods, whereby a rare-earth magnet made of a nanocrystalline magnet is produced. Patent Literature 1 and 2 each disclose examples of such production method.
First, Patent Literature 1 discloses a production method that includes evaporating an evaporation material, which contains at least one of Dy or Tb, onto a molded body that has been obtained through hot plastic processing, and diffusing the evaporation material into the grain boundaries from the surface of the molded body.
This production method requires high-temperature treatment at about 850 to 1050° C. in the step of evaporating the evaporation material. Such a temperature range has been defined so as to improve the residual magnetic flux density and suppress the grain growth at a too high speed.
However, when heat treatment is performed in the temperature range as high as about 850 to 1050° C., the crystal grains will become coarse, which can result in decreased coercivity with high probability. That is, even though Dy or Tb is diffused into the grain boundaries, it becomes consequently impossible to sufficiently increase the coercivity.
Meanwhile, Patent Literature 2 discloses a production method that includes bringing an at least one element selected from Dy, Tb, and Ho, or an alloy containing such element and at least one element elected from Cu, Al, Ga, Ge, Sn, In, Si, P, and Co into contact with the surface of a rare-earth magnet, and diffusing the element or the alloy into the grain boundaries by applying heat treatment such that the grain size will not become greater than 1 μm.
Herein, Patent Literature 2 discloses that when the temperature of the heat treatment is in the range of 500 to 800° C., it is possible to achieve an excellent balance between the effect of diffusion of Dy or the like into the crystal grain boundary phase and the effect of suppressing coarsening of the crystal grains due to the heat treatment, whereby a rare-earth magnet with high coercivity can be easily obtained. In addition, Patent Literature 2 discloses various embodiments in which Dy-Cu alloys are used and heat treatment at 500 to 900° C. is performed. Among the various embodiments, a 85 Dy-15 Cu alloy, which is a representative example, has a melting point of about 1100° C. However, in order to diffuse and infiltrate a metal melt of such an alloy, it would be necessary to perform high-temperature treatment at about 1000° C. or greater. Consequently, it would be impossible to suppress coarsening of the crystal grains.
Thus, since the alloy in Patent Literature 2 when heat treatment in the range of 500 to 800° C. is performed is in the solid phase, and Dy—Cu alloys and the like are diffused into the rare-earth magnet through solid-phase diffusion, it is easily understood that the diffusion takes a long time.
In view of the foregoing various circumstances (e.g., the costs of Dy and the like are increasing; crystal grains will become coarse under a high-temperature atmosphere when a modifying alloy containing a high-melting-point heavy rare-earth element is diffused into the grain boundary phase; and solid-phase diffusion of such a modifying alloy takes a long time), the inventors have arrived at a rare-earth magnet made of a nanocrystalline magnet that contains no heavy rare-earth metals such as Dy or Tb in the grain boundary phase, has high coercivity, in particular, high coercivity under a high-temperature atmosphere, and has relatively high magnetizability, and a method for producing such a magnet.